This invention relates to a process for the coating of polyethylene film on one side of aluminum foil. More particularly, it relates to such a process in which a support material is laminated to the other side of the foil and the foil is formed into a roll for temporary storage after lamination of the support material and before coating of the polyethylene.
It has been found that ingredients in the support material, such as hydrophobic additives encountered in paper making and especially lignin sulfate-sulfite ingredients in paper, will contaminate the opposite side of the aluminum foil when it is rolled for temporary storage after lamination of the foil and before its later priming and extrusion coating with polyethylene. This contamination interferes with wettability, preventing primers of adhesion promoters in aqueous solution from functioning. Similar contamination can also be experienced when the support material laminated to the first side of the foil is material other than paper, such as films of polyvinyl chloride, polyester or regenerated cellulose. Higher-cost support materials such as some clay-coated paper using certain binders in the clay provide little or no difficulties in subsequent extrusion coating of foil that has been rolled. However, less expensive paper is often machine-glazed to provide a good printable surface. Such materials may be treated with release agents such as silicones or stearates. Similar release agents may also be used on support materials other than paper. Release agents and other hydrophobic additives, as well as lignins in paper, may cause problems in the wettability of the foil.
The support materials are generally used as backings for thin aluminum foil in the packaging industry to provide the strength needed for handling. The support materials are also often chosen for their printability, providing a surface which readily accommodates decoration and messages.
Such contamination is not a problem in a tandem extruder operation where a paper laminate is adhered to the first side of the foil by extruded polyethylene and then, without rolling the foil for temporary storage, or rolling it only for a short time such as less than about 2 hours, the second side is coated with an adherence promoter fumarato chromium nitrate and extrusion coated with polyethylene. However, some facilities have only one extruder and need to roll the foil and store it for some time after applying the paper so that the extruder can be switched to apply the polyethylene to the second side. Whatever the reason for rolling and storing the paper-coated foil, materials in the paper contaminate the second side of the foil in the roll and cause inadequate adhesion of the polyethylene through the fumarato chromium nitrate. The contamination seems to limit the wettability and reactivity of the foil by the fumarato chromium nitrate solution.
Adhesive primers commonly used in the extrusion coating and laminating of polyethylene and aluminum are of a polymeric nature and use organic solvents permitting use of shellac wash coats to prevent contamination in rolled paper laminates. The primer dissolves the shellac and makes a good bond. However, aqueous solutions of chromium complexes such as fumarato chromium nitrate, which have very desirable adhesion characteristics, do not dissolve shellac wash coatings. Thus, different means for preventing paper and other contamination in rolls need to be found.
The composition, preparation and use of fumarato chromium nitrate and related chromium (III) coordination complexes which are preferred as adherence promoters in adhering polyethylene to aluminum are described in U.S. Pat. No. 3,787,326 -- Deyrup, assigned to the assignee of the present invention. Other chromium (III) coordination complexes with useful adherence promoting properties are known in the art.
It would be desirable to find a process permitting single-pass extrusion for coating paper on one side of foil, rolling the foil for temporary storage, and then priming the other side of the foil with fumarato chromium nitrate and extrusion coating that side with polyethylene having improved durable adhesion.